Antioxidant



United States Patent ANTIOXIDANT -Paul M. Downey, Gainesville, Fla., assignor'to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware No Drawing. Original application June 10, 1950, Serial No. 167,498, now Patent No. 2,726,277, dated Decemher 6, 1955. Divided and this application February 26, 1954, Serial No. 412,932

1 Claim. (Cl. 260-609) This invention relates to a rubber latex antioxidant and to the method of making the same.

Latex compositions and particularly natural latex normally contain an anticoagulant and an antiseptic or germicide. Additionally it is common practice to add an antioxidant for the rubber. All of these ingredientsare loosely referred to as preservatives but the present invention is concerned only with the antioxidant type of preservat-i ve. In the manufacture of articles from a latex rubber the compounding and vulcanizing ingredients including the antioxidant must be added to the matrix before-the rubber is coagulated. Many otherwise suitable antioxidants cannot be used in latex because of some adverse effect on the system such as discoloring or destabilizing the dispersion. For example, certain phenolic sulfides are valuable non-d'iscoloring antioxidants for massed rubber but possess theunfortunate disadvantage of thickening latex and some even bring about immediate coagulation. The present invention provides a class of non-discoloring phenolic sulfide antioxidants which are eminently suitable for addition to an aqueous dispersion of a rubber.

In accordance with the present invention it has been discovered that thiobis (-4,6-dialkyl phenols) containing at least four but less than eight carbon atoms in each, alkyl .group are effective non-discoloring antioxidants for rubber which do not have any adverse effect on the stability -.of the latex .but .on the contrary maybe added to the latex without thickening or coagulating it. It was found that the size, type, number and position of the alkyl groups all influence the compatibility with latex. These prod- .ucts .may :be prepared'by reacting a 4,6-dialkyl phenol with a sulfur halide. The sulfur chlorides are convenient toulse and are readily available for this purpose. Con- ;densation of two molecular -.equivalents of the phenol with one molecular equivalent of sulfur dichloride produces a monosulfide or mono thio bis-phenol. Additional sulfur may be introduced into the molecule by increasingthe ratio of sulfur dichloride or by substituting sulfur monochloride for sulfur dichloride. While the antioxidant effectiveness is not increased by increasing the sulfur content of the, molecule, neither is it noticeably decreased untilthe sulfur ratio is greater than two atoms of sulfur for two dialkylphenol radicals. Whether or not a disul- "fi'd e "linkage forms with higher sulfur content is not definitely known but the experimental evidence indicates as more probable that polymers are formed having more than'two phenol groups linked together by sulfur atoms. 'Thus the composite reaction products of either sulfur "monochloride or sulfur dichloride probably contain some polymericconstituentsand these reaction products are suitable for incorporating into latex without further purific'at'ion than-to remove by-product hydrogen chloride. The sulfur would be expected to attach to the nucleus at -thefree =ortho position and additional sulfur bridges lwould have to join at :one of the meta positions. The atthinbis(4-,,6-dialkyl;phenol)-is used in v,a generic sense to include the sulfur halide reaction products-of 2,776,998 Patented Jan. 8, 1957 4,6-dialkyl phenols generally. The ratio of the phenol radical to sulfur may vary and includes 2:1, 1:1 and even higher sulfur ratios.

The reactions may be carried out in a solvent unaffected by a sulfur halide or preferably in the absence of a solvent by direct reaction between the phenol and sulfur halide. It is unnecessary to employ pure fractions of the dialkyl phenols. Small amounts of other isomers providing they are not present in more than a few percent have not proved deleterious. Thus it is probable that small amounts of 2,6-dialkyl phenols are present in the commercial grades of 2,4-dialkyl phenols although these commercial fractions have proved to be satisfactory without further purification. The length of the alkyl chain isqa critical factor as compounds with less than four or more than seven carbon atoms in the alkyl groups cause rapid coagulation of latex. Thiobis(4,6-di-sec0ndary amyl phenol) is about optimum and is preferred. It is absolutely non-discoloring and produces a more stable latex than thiobis(2,4-di-tertiary amyl phenol), which latter causes some, however slight, discoloration of white rubber goodsand exhibits less antioxidant strength. So far as is known thiobis(4,6-di-secondary amyl phenol) has never before been described.

Without limiting the invention the following are recited to illustrate the preparation of the new antioxidants.

, EXAMPLE 1 much more fluid than thiobis(4,6-di-tertiary amyl phenol) obtained by substituting 2,4-diamyl phenol in which the alkyl groups were predominantly tertiary for 2,4-di-secondar-y amyl phenol in the foregoing procedure.

EXAMPLE 2 The reaction product of sulfur monochloride was-prepared by substituting 77.0 grams of sulfur monochloride for the 58.8 grams of sulfur dichloride in the foregoing example. After removal of the volatile constituents as described, there was obtained a resinous product containing substantially one atom of sulfur for each 2,4-di-secaondary amyl phenol radical.

The critical differences brought about by small changes in structure "maybe readily demonstrated by adding the antioxidant to the latex and following the change in viscosity with time. For example a typical dipped goods base stock was compounded comprising Parts by weight Rubber as 60% latex 100.0

Zinc oxide 1.0 Sulfur 1.5 Zinc diethyl dithiocarbamate 0.5

Antioxidant 1.0

The stocks were compounded 1n the manner common to the art by adding zinc oxide, sulfur and accelerator in the form of dispersions to the concentrated latex. The antioxidants were also added as dispersions, the following procedure proving satisfactory for the preparation of such an antioxidant dispersion: parts of the antioxidant were mixed with 10 parts of a 10% solution of ammonium :caseina-te with moderate stirring and I4 parts .of .oleic acid were then introduced and after mixing about minutes, 4 parts of aqueous NaOH was added and finally 82 parts of water added very slowly.

The viscosities of the latex were determined after com pounding and at intervals thereafter. For this purpose It will be noted that the antioxidants have no adverse effect on the cure and are etficient antioxidants.

In addition, samples of the cured stocks were exposed to the rays of an S-1 sunlamp. After ten days exposure a Brookfield viscometer was used. The data set forth 5 there was no noticeable difference between the color of in the table below are relative figures reported as perexposed stocks A and C. Stock B was only slightly centages of the control stock which contained no antidarker. In order to obtain a more precise measure of oxidant. In other words the control stock was arbitrarily the color variation, the light reflected from the surface assigned a value of 100. of the stocks was measured by means of a photovolt re- Table I Relative viscosity after- Antioxidant Ohrs. 24hrs. 48 hrs. 72hrs. 96 hrs. 120 hrs. 240 hrs. 404 hrs. 600 hrs.

None 100 100 100 100 100 100 100 100 100 Thiobis (to dl-isopropylphenol) 80 500 1,270 coag. Thiobis (4,6-di-tertiary butyl phenol) 79 51 vise. coag. Thiobis (4,6-di-tertiary arnyl p en 72 65 78 81 86 134 Thiobis (tool-secondary amyl phenol c7 55 69 7a 77 10s Thiobis (4,6'di-tertiary octyl phenol) 79 coag.

Stock A B 0 Parts by WeightDry Basis giixllbbcr as 60% latex 100 100 100 Sulfur Zinc diethyl dithiocarbamate- Titanium dioxide Thiobis (4,6-di-tertiary amyl phenol Thiobis (4,6-di-secondary amyl pheno Films were cast on glass from the latex compounds so prepared and were stripped after about 12 hOlll's and dried at room temperature in circulating air. After about 18 hours drying the films changed from opaque to translucent indicating dryness and were then cured by heating for minutes at 100 C. in circulating air. Before testing, the dry films whether cured or uncured, were conditioned by holding at constant temperature in a desiccator over sulfuric acid for at least 24 hours. The tensile properties were then measured in the usual manner.

Aging tests were carried out by heating samples of the compounds 9 hours in a bomb at 121 C. under 80 pounds air pressure per square inch.

Table III Percent Light R3- flectance After 10 days Exposure To Ultra Violet Light Stock Obi Similar tests were carried out in latex sponge stocks comprising- Parts by weight Rubber as 60% latex 100.0 Zinc diethyl dithiocarbamate 1.0

Piperidine cyclopentamethylene dithiocarbamate 0.25 Sulfur 3.0 KOH 0.25 Zinc oxide 6.0

Antioxidant These ingredients except the zinc oxide and antioxidant were compounded in known manner and 300 gram portions of the stock frothed using 10 grams of 10% ammonium oleate as a dispersing aid. By means of a high speed mixer they were frothed to six times their original volume and the speed of the whip reduced to permit the froth to smooth out before adding the antioxidant. After 5 minutes of mixing, the antioxidant and zinc oxide were introduced and permitted to mix for 3 minutes. 10 grams of 20% sodium silicofiuoride was then added and the spongy mass removed to the curing molds and the sponge cured by heating for 60 minutes in water at C. Samples of the cured product were exposed to ultra violet light employing a General Electric S-l lamp as the light source and the percent light reflectance determined as described above.

ya-n.

Table IV Percent Light Reflectance after Exposure to 8-1 lamp (days) Antioxidant None 76 76 73 62 62 Thiobis (4,0-di-tertiary arnyl phenol) m 76 71 69 64 54 Thiobis (4,6-di-secondary amyl phenol. 75 75 72 68 66 Similar desirable properties are exhibited in synthetic rubber compositions, as for example GR-S latex type III.

In general amounts Within the range of 0.5% to 3.0% based on the total elastomer solids are preferred. While 1.0% is the amount usually employed even better protection may be achieved by increasing the concentration to 2.0%. The eifect of increasing quantities gradually diminishes. The antioxidant properties are of course similar regardless of how the antioxidants are incorporated into the rubber or elastomer. Thus the method of treat- 20 References Cited in the file of this patent UNITED STATES'PATENTS 2,139,321 Mikeska et al. Dec. 6, 1938 2,270,183 Cook et al. Jan. 13, 1942 2,425,824 Peters et al Aug. 19, 1947 2,472,517 Cantrell et al. June 7, 1949 

